BackupableDB delete backups with newer seq number

Summary: We now delete backups with newer sequence number, so the clients don't have to handle confusing situations when they restore from backup.

Test Plan: added a unit test

Reviewers: dhruba

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D14547
[RocksDB perf] Cache speedup

Summary:
I have ran a get benchmark where all the data is in the cache and observed that most of the time is spent on waiting for lock in LRUCache.

This is an effort to optimize LRUCache.

Test Plan:
The data was loaded with fillseq. Then, I ran a benchmark:

    /db_bench --db=/tmp/rocksdb_stat_bench --num=1000000 --benchmarks=readrandom --statistics=1 --use_existing_db=1 --threads=16 --disable_seek_compaction=1 --cache_size=20000000000 --cache_numshardbits=8 --table_cache_numshardbits=8

I ran the benchmark three times. Here are the results:
AFTER THE PATCH: 798072, 803998, 811807
BEFORE THE PATCH: 782008, 815593, 763017

Reviewers: dhruba, haobo, kailiu

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D14571
When flushing mem tables, create iterators out of mutex

Summary:
creating new iterators of mem tables can be expensive. Move them out of mutex.
DBImpl::WriteLevel0Table()'s mems seems to be a local vector and is only used by flushing. memtables to flush are also immutable, so it should be safe to do so.

Test Plan: make all check

Reviewers: haobo, dhruba, kailiu

Reviewed By: dhruba

CC: igor, leveldb

Differential Revision: https://reviews.facebook.net/D14577

Conflicts:
	db/db_impl.cc
[RocksDB Performance Branch] Avoid sorting in Version::Get() by presorting them in VersionSet::Builder::SaveTo()

Summary: Pre-sort files in VersionSet::Builder::SaveTo() so that when getting the value, no need to sort them. It can avoid the costs of vector operations and sorting in Version::Get().

Test Plan: make all check

Reviewers: haobo, kailiu, dhruba

Reviewed By: dhruba

CC: nkg-, igor, leveldb

Differential Revision: https://reviews.facebook.net/D14409
Introduce MergeContext to Lazily Initialize merge operand list

Summary: In get operations, merge_operands is only used in few cases. Lazily initialize it can reduce average latency in some cases

Test Plan: make all check

Reviewers: haobo, kailiu, dhruba

Reviewed By: haobo

CC: igor, nkg-, leveldb

Differential Revision: https://reviews.facebook.net/D14415

Conflicts:
	db/db_impl.cc
	db/memtable.cc